1. Field of the Invention
The present invention generally relates to a coherent detecting method and, more particularly, to a coherent detecting method which performs a detection of synchronism by estimating a propagation characteristic of a propagation path by using a pilot symbol and a data symbol.
2. Description of the Related Art
In a mobile communication system, when a terminal moves under a condition in which a multipath is formed, fluctuation is generated in a receiving signal due to fading. When the received signal is decoded under such a condition, a method for performing a synchronism detection is generally used, in which method a propagation characteristic (a fading complex path) is estimated from a received pilot symbol so as to reduce influences of fading caused by a propagation characteristic of the propagation path.
The above-mentioned pilot symbol is a known symbol having a predetermined amplitude and a predetermined phase. The pilot symbol can be transmitted from a transmitter by being provided in a data frame of data symbols. Alternatively, the pilot symbol may be transmitted through a channel different from a channel through which the data symbols are transmitted.
When the pilot symbol is provided in the data string of the data symbols, the pilot symbol is inserted at a predetermined position of the data frame to be transmitted. On the receiver side, the position of the pilot symbol is detected according to synchronization with a preamble added to the data frame. The detected pilot symbol is demodulated so as to assume or estimate a characteristic of the propagation path from an amplitude and a phase of the detected pilot symbol.
A description will now be given of a conventional receiver including a coherent detecting circuit using a pilot symbol provided in a data string of data symbols.
FIG. 1 is an illustration of a data frame provided with pilot symbols. FIG. 1-(A) shows a data frame to be transmitted, and FIG. 1-(B) shows data symbol strings and the pilot symbols in encoded data. As shown in FIG. 1-(A), the data frame 12-1 comprises a preamble 12-2 and the encoded data 12-3. The encoded data includes, as shown in FIG. 1-(B), a plurality of data symbol strings 12-4 and a plurality of pilot symbols 12-5. A plurality of pilot symbols are provided between adjacent data symbol strings 12-4 at uniform intervals. The portion in which the pilot symbols are provided is referred to as a pilot block 12-6.
The pilot symbol 12-5 is a previously determined, known data symbol. The pilot symbol 12-5 is transmitted at a predetermined time interval after the preamble 12-2 is transmitted by a transmitter. If synchronism is achieved between a receiver and a receiving signal transmitted by the transmitter and received via a propagation path, the receiver can assume a propagation characteristic of the propagation path from the received signal with respect to a time position.
The receiver can synchronize with the receiving signal by detecting the preamble 12-2 provided in the data frame 12-1.
Now, a pilot symbol which is the k-th pilot symbol in the n-th pilot block transmitted by the transmitter is represented by Znk, and a propagation characteristic at the time the pilot signal is transmitted is represented by .xi.nk. A receiving symbol received via the propagation path is represented by Znk.multidot..xi.nk.
Since a transmitted symbol at the time position is the pilot symbol Znk which is a known symbol, .xi.nk.multidot..vertline.Znk.vertline..sup.2 is obtained by multiplying the receiving symbol by Znk* which is a complex conjugate of the pilot symbol Znk. Since a magnitude (amplitude) of the pilot symbol is a known value (may be .vertline.Znk.vertline..ident.1), the propagation characteristic .xi.nk of the propagation path can be estimated.
The estimated value .xi.nk of the propagation characteristic can be represented as follows. EQU .xi.nk =Znk.multidot..xi.nk.multidot.Znk*=.xi.nk.multidot..vertline.Znk.vertline. .sup.2 (1)
However, in practice, since the receiving symbol is influenced by interference caused by noise and other signals, the propagation characteristic of the propagation path cannot be accurately estimated. In order to more accurately assume the propagation characteristic, a plurality of pilot symbols may be provided in a single pilot block 12-6 so as to obtain an estimated value of the propagation characteristic for each of the pilot symbols. An average value of the estimated value of the propagation characteristic is determined to be the estimated value of the propagation characteristic of the propagation path. It is estimated that an estimated value of the propagation characteristic of the n-th pilot block is represented by .xi.n .
The propagation characteristic of the propagation path between two pilot blocks 12-6 can be obtained by averaging propagation characteristics at the positions of the two pilot blocks 12-6 or performing a liner interpolation on the propagation characteristics.
After the estimated value of the propagation characteristic of the propagation path is obtained, the transmitted data symbol is obtained as follows. It is estimated that the i-th transmitted data symbol between the n-th pilot block and the (n+1)-th pilot block is represented by Xni; an actual value of the propagation characteristic of the propagation path is represented by .xi.ni; an estimated value of the propagation characteristic of the propagation path is represented by .xi.ni ; and a decoded data symbol is represented by Xni .
The received data symbol transmitted through the propagation path is represented by Xni.multidot..xi.ni, which is obtained by multiplying the transmission data symbol Xni by the actual value .xi.ni of the propagation characteristic of the propagation path. The transmission data symbol Xni can be obtained as the decoded data symbol Xni , in which the influence of the propagation characteristic .xi.ni is reduced, by multiplying Xni.multidot..xi.ni by the complex conjugate .xi.ni * of the estimated value .xi.ni of the propagation characteristic. The decoded data symbol Xni is represented by the following equation. EQU Xni =Xni.multidot..xi.ni.multidot..xi.ni */.vertline..xi.ni .vertline..sup.2 (2)
The thus-obtained decoded data symbol is subjected to a diversity-combining and is determined as a predetermined discrete data symbol when compared with a predetermined threshold value by a determining circuit. Thereafter, the decoded data symbol is subjected to a decoding process such as deinterleaving or a bit error correction so as to be reproduced as data. FIG. 2 shows a receiver including a conventional coherent detecting circuit using pilot symbols provided in data symbols.
In FIG. 2, a signal received by an antenna (ANT) 13 is input to a radio unit 14. In the radio unit 14, the received signal is amplified by an amplifier (LNA) 14-1, and all but a component of a predetermined bandwidth is eliminated by a band-pass filter (BPF) 14-2. Additionally, the received signal is converted into a base bandwidth by being multiplied by a signal LO in a mixer 14-3. Then, a high-frequency component is removed by a low-pass filter (LPF) 14-4, and the received signal is output to an A/D circuit 15.
The A/D circuit 15 quantizes the received signal supplied by the radio unit 14 so as to convert the received signal into a digital signal, and outputs the digital signal to a timing synchronizing circuit 16. The timing synchronizing circuit 16 performs a synchronization by using the digitized received signal, and outputs the digitized received signal to a coherent detecting circuit 17.
In the coherent detecting circuit 17, a propagation path estimation circuit 17-1 calculates the estimated value .xi.ni of the propagation characteristic of the propagation path based on the above-mentioned equation (1) so as to obtain a complex conjugate .xi.ni * of the estimated value .xi.ni . Then, the thus-obtained complex conjugate .xi.ni * is output to a multiplier 17-3.
The multiplier 17-3 performs a synchronism detection by multiplying the received signal, after passing through a delay circuit 17-2, by the complex conjugate .xi.ni * output from the propagation path estimation circuit 17-1. The decoded data symbol Xni is output to a diversity-combining circuit 17-4.
It should be noted that, in the above-mentioned coherent detecting circuit 17, The decoded data symbol Xni is calculated by multiplying the received signal Xni.multidot..xi.ni by the complex conjugate .xi.ni * of the estimated value of the propagation characteristic whereas the multiplication should be performed by using .xi.ni */.vertline..xi.ni .vertline..sup.2. However, since the calculation of .vertline.86 ni .vertline..sup.2 influences only an amplitude component of the decoded data symbol Xni , the multiplication performed by the multiplier 17-3, that is, the multiplication by the complex conjugate .xi.ni * of the estimated value of the propagation characteristic, can be used as a substitute.
The decoded data symbol Xni is diversity-synthesized by the diversity-combining circuit 17-4 with decoded data symbols obtained by other similar circuits. The synthesized decoded data symbol is determined as a predetermined discrete data symbol by being compared with a predetermined threshold value by a determining circuit 17-5, and is output to a decoder 18.
As can be appreciated from the above-mentioned equation (2), if the actual value .xi.ni of the propagation characteristic and the estimated value .xi.ni of the propagation characteristic are equal to each other, the decoded data symbol Xni coincides with the transmitting data symbol Xni. However, if a difference between the actual value .xi.ni and the estimated value .xi.ni of the propagation characteristic of the propagation path is large, a difference between the decoded data symbol Xni and the transmitting data symbol Xni is also large.
Accordingly, it is important to accurately assume the actual value of the propagation characteristic of the propagation path so as to accurately decode the data symbol. One of the methods to increase accuracy of the assumption of the propagation characteristic is to increase the number of pilot symbols to be provided in the pilot block 12-6. However, if the number of pilot symbols is increased, a data transmission efficiency is deteriorated since the number of data symbols to be transmitted is decreased.
Additionally, if there is a large difference in the propagation characteristic between the pilot blocks, that is, if a fading frequency is high, an estimating method using a pilot symbol having a fixed period cannot accurately assume the propagation characteristic of a data symbol of a data symbol string positioned between the pilot blocks in response to the fading frequency.
Generally, when the fading frequency is low, an accuracy of an average value of the propagation characteristic is higher than an accuracy of a value obtained by an interpolation in an area in which a value of a normalized fading frequency fd.multidot.Tp is smaller than about 0.1, where fd is a maximum Doppler frequency and Tp is a period of the pilot blocks.
On the other hand, if the normalized fading frequency fd.multidot.Tp is high, the interpolation is superior to the averaging method in an area where the normalized fading frequency fd.multidot.Tp is greater than about 0.1. However, even if the interpolation is adopted, the accuracy of assumption for an area in which the normalized fading frequency fd.multidot.Tp is high is deteriorated as compared to that of an area in which the normalized fading frequency fd.multidot.Tp is low. As a result, a data error rate is increased.
Additionally, as mentioned above, a pilot symbol may be transmitted through a channel different from a channel through which a data symbol is transmitted. That is, the pilot symbol may be multiplexed with the data symbol by being transmitted through a channel that is orthogonal to the channel of the data symbol. Since the pilot symbol is concurrently transmitted with the data symbol, this method is referred to as a concurrent pilot-channel method.
In this method, the data symbol and the pilot symbol are multiplied by codes orthogonal to each other so as to modulate the data symbol and the pilot symbol according to an I-channel and a Q-channel, respectively. This method is particularly referred to as a pilot IQ multiplex method.
Since the synchronism detection with the thus-provided pilot symbol includes a process for demultiplexing the pilot symbol and the data symbol, an application for a mobile communication field by a Direct Sequence Code Division Multiple Access (DS-CDMA) has been considered.
In this method, since the pilot symbol and the data symbol are orthogonal to each other, those symbols can be separated from each other after a demodulation on the receiver side. The separated symbol is used for estimating a characteristic of the propagation path, and the received data symbol can be subjected to an accurate synchronism detection based on the estimated propagation characteristic.
An assumption or estimation of a characteristic of the propagation path can be performed in the same manner as described above.
That is, an estimated value .xi.n of the propagation characteristic corresponding to the n-th pilot symbol Zn can be represented as follows. EQU .xi.n =Zn.multidot..xi.n.multidot.Zn*=.xi.n.multidot..vertline.Zn.vertline..sup. 2 (3)
Additionally, a decoded data symbol Xn can be represented by the following equation. EQU Xn =Xn.multidot..xi.n.multidot..xi.n */.vertline..xi.n .vertline..sup.2 (4)
FIG. 3 shows a receiver including a conventional coherent detecting circuit 19 using pilot symbols concurrently transmitted with data symbols. In FIG. 3, parts that are the same as the parts shown in FIG. 2 are given the same reference numerals, and descriptions thereof will be omitted.
The coherent detecting circuit 19 of the receiver shown in FIG. 3 has the same construction as the coherent detecting circuit 17 shown in FIG. 2 except for multipliers 19-1 and 19-2 being added. That is, the multiplier 19-1 multiplies the received signal by an orthogonal code X so as to extract a received data symbol, and the multiplier 19-2 multiplies the received signal by an orthogonal code Y so as to extract a received pilot symbol. The orthogonal codes X and Y are orthogonal to each other. It should be noted that the data symbol and the pilot symbol in the received signal have been modulated and multiplexed according to the orthogonal codes X and Y.
The received data symbol output from the multiplier 19-1 is input to the delay circuit 17-2. The received pilot data is input to the propagation path estimation circuit 17-1.
In this estimating method, it is considered to increase a transmission power of the pilot channel so as to increase an accuracy of assumption of the characteristic of the propagation path. However, an increase in the transmission power of the pilot channel may increase a power consumption of a mobile communication terminal. In such a case, in order to maintain the total power consumption of the mobile communication terminal, power consumption for the data channel must be decreased. However, this results in deterioration in an S/N ratio of the data symbol. Additionally, an increase in the transmission power for the pilot channel causes an interference with the data channel. This results in deterioration of a channel capacity characteristic in a DS-CDMA mobile communication.
Additionally, It is a consideration to increase a number of pilot symbols so as to increase an accuracy of assumption of the characteristic of the propagation path. However, when fluctuation in the characteristic of the propagation path relative to an averaging section of the pilot symbols is large, the accuracy may be deteriorated. Thus, the number of pilot symbols that can be averaged has an upper limit.